1. Field of the Invention
This invention relates to a conductive member, and more particularly to a conductive member used in a contact charging assembly, which is disposed in contact with an electrophotographic photosensitive member and to which a voltage is applied to charge the surface of the electrophotographic photosensitive member to a stated potential.
2. Related Background Art
A number of methods for electrophotography are conventionally known. Copies are commonly obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a visible image (toner image), transferring the toner image to a transfer medium such as paper as the occasion demands, and thereafter fixing the toner image to the transfer medium by heat and/or pressure. Toner particles that have not transferred to the transfer medium and remain on the photosensitive member are removed from the photosensitive member through a cleaning step.
Corona charging assemblies have conventionally been used as charging assemblies for electrophotography. In recent years, in place of these, contact charging assemblies have been put into practical use. This aims at low ozone and low power consumption. In particular, a roller charging system making used of a conductive roller as a charging member is preferably used in view of the stability of charging.
In such roller charging, an elastic roller is brought into contact with a member to be charged and a voltage is applied thereto to charge the member electrostatically.
Stated specifically, the charging is performed by the release of electric energy from the charging member to the member to be charged, and hence the charging begins upon application of a voltage not lower than a certain threshold voltage. For example, when a charging roller is brought into pressure contact with an organic photoconductor electrophotographic photosensitive member (OPC electrophotographic photosensitive member) having a photosensitive layer of 25 xcexcm thick, the surface potential of the electrophotographic photosensitive member begins to rise upon application of a voltage of about 640 V or higher as absolute value, and then, the surface potential of the electrophotographic photosensitive member increases linearly at a slope of 1 with respect to the applied voltage. Hereinafter, this threshold voltage is defined as charging start voltage Vth.
Namely, in order to attain the surface potential Vd of an electrophotographic photosensitive member that is considered necessary for electrophotography, a DC voltage of Vd+Vth, which is beyond the level considered necessary for the image formation itself, is required for the charging roller. This method, in which only a DC voltage is applied to the contact charging member in this way to perform the charging, is called DC charging.
In the DC charging, however, it has been difficult to keep the potential of the electrophotographic photosensitive member at a desired value because the electrical resistance value of the contact charging member tends to vary depending on environmental variations and also because any change in layer thickness as a result of abrasion of the electrophotographic photosensitive member may cause variations of the Vth.
Accordingly, in order to make the charging much more uniform, as disclosed in Japanese Patent Application Laid-open No. 63-149669, an AC+DC charging system is used in which a voltage formed by superimposing on a DC voltage corresponding to a desired Vd an AC component having a peak-to-peak voltage of 2xc3x97Vth or higher is applied to the contact charging member. This aims at the effect of leveling the potential by AC, where the potential of the member to be charged converges at the Vd that is the middle of a peak of AC voltage, and can be affected with difficulty by any external disorder such as environmental variations.
As conductive members used for charging, U.S. Pat. No. 4,967,231 discloses an example in which a conductive seamless tube is used to form a surface layer on a conductive support member. Also, Japanese Patent Application Laid-open No. 5-2313 discloses a seamless tube comprised of a fluorine resin, and Japanese Patent Application Laid-open No. 5-96648 discloses a multi-layer tube constituted of layers having different conductivities. As methods concerning the production of charging members, the above prior art U.S. Patent teaches a method of forming the surface layer by inserting the support member into the seamless tube. Japanese Patent Application Laid-open No. 6-58325 also discloses a method of forming a surface layer by the use of a cross-head extruder.
Such methods of forming a roller layer by using the seamless tube enable more uniform changing to be performed with ease because, even when a foam is used as an elastic layer formed on a substrate, a smooth surface can be formed by further covering it with the seamless tube.
As a method of endowing the seamless tube with conductivity, it may commonly include a method of ion conduction which uses salt as a conducting agent and a method of electron conduction which uses a conducting substance such as carbon black or conductive metal oxide powder as a conducting agent. In the case when the seamless tube is endowed with conductivity by ion conduction, the electrical resistance value tends to greatly vary depending on environment. Also, there is such a problem that, since the seamless tube comes into contact with the electrophotographic photosensitive member, the salt tends to contaminate the photosensitive member.
However, in the case when the conducting substance such as carbon or conductive metal oxide powder is incorporated in an insulating material, there has been such a disadvantage that the electrical resistance tends to increase because of repetitive electrification. Especially when the electrical resistance is to be kept low, the conducting agent must be incorporated in a large quantity. Incorporating the conducting agent in a large quantity makes it easy to control the increase in electrical resistance. However, when the electrical resistance is kept at a medium resistance of about 1xc3x97104 to 1xc3x971011 xcexa9xc2x7cm, which is required for the covering layer of the conductive member, there is also a limit of the quantity of the conducting agent. Thus, it has been difficult to well control the increase in electrical resistance due to electrification.
An object of the present invention is to provide a conductive member which may cause, even when successively electrified, less variations in the value of electrical resistance in the medium resistance region (1xc3x97104 to 1xc3x971011 xcexa9xc2x7cm), has less scattering of the electrical resistance value and is superior in production stability.
Another object of the present invention is to provide a process cartridge and an electrophotographic apparatus which have such a conductive member.
The present invention provides a conductive member which is to be disposed in contact with an electrophotographic photosensitive member and to which a voltage is to be applied; the conductive member comprising a support member and a conductive covering layer provided on the support member;
the conductive covering layer comprising both a first carbon black having a DBP oil absorption of from 300 cm3/100 g to 500 cm3/100 g and a second carbon black having a DBP oil absorption of 250 cm3/100 g or smaller and exhibiting the pH of 5 or below.
The present invention also provides a process cartridge and an electrophotographic apparatus which have the above conductive member.